The genomes of eukaryotic organisms are highly organized within the nucleus of the cell. The long strands of duplex DNA are wrapped around an octomer of histone proteins to form a nucleosome. This basic unit is then further compressed by the aggregation and folding of nucleosomes to form a highly condensed chromatin structure. A range of different states of condensation are possible, and the tightness of this structure varies during the cell cycle, being most compact during the process of cell division. There has been appreciation recently that chromatin templates form a fundamentally important set of gene control mechanisms referred to as epigenetic regulation. By conferring a wide range of specific chemical modifications to histones and DNA (such as acetylation, methylation, phosphorylation, ubiquitinylation and SUMOylation) epigenetic regulators modulate the structure, function and accessibility of our genome, thereby exerting a huge impact in gene expression.
Histone acetylation is most usually associated with the activation of gene transcription, as the modification loosens the interaction of the DNA and the histone octomer by changing the electrostatics. In addition to this physical change, specific proteins bind to acetylated lysine residues within histones to read the epigenetic code. Bromodomains are small (˜110 amino acid) distinct domains within proteins that bind to acetylated lysine resides commonly but not exclusively in the context of histones. There is a family of around 50 proteins known to contain bromodomains, and they have a range of functions within the cell. The BET family of bromodomain containing proteins comprises 4 proteins (BRD2, BRD3, BRD4 and BRD-T) which contain tandem bromodomains capable of binding to two acetylated lysine residues in close proximity, increasing tile specificity of the interaction.
BRD2 and BRD3 are reported to associate with histones along actively transcribed genes and may be involved in facilitating transcriptional elongation (Leroy et al, Mol. Cell. 2008 30(1):51-60), while BRD4 appears to be involved in the recruitment of the pTEF-13 complex to inducible genes, resulting in phosphorylation of RNA polymerase and increased transcriptional output (Hargreaves et al, Cell, 2009 138(1): 1294145). All family members have been reported to have some function in controlling or executing aspects of the cell cycle, and have been shown to remain in complex with chromosomes during cell division—suggesting a role in the maintenance of epigenetic memory. In addition some viruses make use of these proteins to tether their genomes to the host cell chromatin, as part of the process of viral replication (You et al Cell, 2004 117(3):349-60).
Recent articles relating to this target include Prinjha et al., Trends in Pharmacalogical Sciences, March 2012, Vol. 33, No. 3, pp. 146-153; Conway, ACS Med. Chem. Lett., 2012, 3, 691-694 and Hewings et al., J. Med. Chem., 2012, 55, 9393-9413.
Small molecule BET inhibitors that are reported to be in development include GSK-525762 A, OTX-015 as well as others from the University of Oxford and Constellation Pharmaceuticals Inc.
Hundreds of epigenetic effectors have been identified, many of which are chromatin-binding proteins or chromatin-modifying enzymes. These proteins have been associated with a variety of disorders such as neurodegenerative disorders, metabolic diseases, inflammation and cancer. Thus, these compounds which inhibit the binding of a bromodomain with its cognate acetylated proteins, promise new approaches in the treatment of a range of autoimmune and inflammatory diseases or conditions and in the treatment of various types of cancer.